The present invention relates generally to a thermal imaging system and, more particularly, to a multicolor thermal imaging system wherein at least two image-forming layers of a thermal imaging member are addressed at least partially independently by a single thermal printhead or by multiple printheads from the same surface of the thermal imaging member.
Conventional methods for color thermal imaging such as thermal wax transfer printing and dye-diffusion thermal transfer typically involve the use of separate donor and receiver materials. The donor material typically has a colored image-forming material, or a color-forming imaging material, coated on a surface of a substrate and the image-forming material or the color-forming imaging material is transferred thermally to the receiver material. In order to make multicolor images, a donor material with successive patches of differently-colored, or different color-forming, material may be used. In the case of printers having either interchangeable cassettes or more than one thermal head, different monochrome donor ribbons are utilized and multiple color separations are made and deposited successively above one another. The use of donor members with multiple different color patches or the use of multiple donor members increases the complexity and the cost of such printing systems. It would be simpler to have a single-sheet imaging member that has the entire multicolor imaging reagent system embodied therein.
There have been described in the prior art numerous attempts to achieve multicolor, direct thermal printing. For example, there are known two-color direct thermal systems in which formation of the first color is affected by formation of the second color. U.S. Pat. No. 3,895,173 describes a dichromatic thermal recording paper which includes two leuco dye systems, one of which requires a higher activation temperature than the other. The higher temperature leuco dye system cannot be activated without activating the lower temperature leuco dye system. There are known direct thermal imaging systems that utilize an imaging member having two color-forming layers coated on opposite surfaces of a transparent substrate. The imaging member is addressed by multiple printheads independently from each side of the imaging member. A thermal imaging system of this type is described in U.S. Pat. No. 4,956,251.
Thermal systems that exploit a combination of dye transfer imaging and direct thermal imaging are also known. In systems of this type, a donor element and a receiver element are in contact with one another. The receiver element is capable of accepting dye, which is transferred from the donor element, and also includes a direct thermal color-forming layer. Following a first pass by a thermal printhead during which dye is transferred from the donor element to the receiver element, the donor element is separated from the receiver and the receiver element is imaged a second time by a printhead to activate the direct thermal imaging material. This type of thermal system is described in U.S. Pat. Nos. 4,328,977. 5,284,816 describes a thermal imaging member that comprises a substrate having a direct thermal color-forming layer on one side and a receiver element for dye transfer on the other side.
There are also known thermal imaging systems that utilize imaging members having spatially separated regions comprising direct thermal color-forming compositions that form different colors. U.S. Pat. Nos. 5,618,063 and 5,644,352 describe thermal imaging systems in which different areas of a substrate are coated with formulations for forming two different colors. A similar bicolored material is described in U.S. Pat. No. 4,627,641.
Another known thermal imaging system is a leuco-dye-containing, direct thermal system in which information is created by activating the imaging material at one temperature and erased by heating the material to a different temperature. U.S. Pat. No. 5,663,115 describes a system in which a transition from a crystalline to an amorphous, or glass, phase is exploited to give a reversible color formation. Heating the imaging member to the melting point of a steroidal developer results in the formation of a colored amorphous phase while heating of this colored amorphous phase to a temperature lower than the crystalline melting point of the material causes recrystallization of the developer and erasure of the image.
There is also known a thermal system containing one decolorizable, leuco dye containing, color-forming layer and a second leuco dye containing layer capable of forming a different color. The first color-forming layer colorizes at a low temperature while the second layer colorizes at a higher temperature, at which temperature the decolorization of the first layer also takes place. In such systems, either one or the other color can be addressed at a particular point. U.S. Pat. No. 4,020,232 discloses formation of one color by a leuco dye/base mechanism and the other by a leuco dye/acid mechanism wherein the color formed by one mechanism is neutralized by the reagent used to form the other. Variations of this type of system are described in U.S. Pat. Nos. 4,620,204; 5,710,094; 5,876,898 and 5,885,926.
Direct thermal imaging systems are known in which more than one layer may be addressed independently, and in which the most sensitive color-forming layer overlies the other color-forming layers. Following formation of an image in the layer outermost from the film base, the layer is deactivated by exposure to light prior to forming images in the other, less sensitive, color-forming layers. Systems of this type are described in U.S. Pat. Nos. 4,250,511; 4,734,704; 4,833,488; 4,840,933; 4,965,166; 5,055,373; 5,729,274; and 5,916,680.
As the state of the thermal imaging art advances and efforts are made to provide new thermal imaging systems that can meet new performance requirements, and to reduce or eliminate some of the undesirable requirements of the known systems, it would be advantageous to have a muticolor thermal imaging system in which at least two different image-forming layers of a single imaging member can be addressed at least partially independently from the same surface by a single thermal printhead or by multiple thermal printheads so that each color can be printed alone or in selectable proportion with the other color(s).
It is therefore an object of this invention to provide a multicolor thermal imaging system which allows for addressing, at least partially independently, with a single thermal printhead or multiple thermal printheads, at least two different image-forming layers of an imaging member from the same surface of the imaging member.
Another object of the invention is to provide such a multicolor thermal imaging system wherein each color can be printed alone or in selectable proportion with the other color(s).
Yet another object of the invention is to provide a multicolor thermal imaging system wherein at least two different image-forming layers of an imaging member are addressed at least partially independently by controlling the temperature applied to each of the layers and the time each of the layers is subjected to such temperature.
Still another object of the invention is to provide a multicolor thermal imaging system wherein at least two different image-forming layers of an imaging member are addressed at least partially independently with a thermal printhead or multiple thermal printheads from the same surface of the imaging member and one or more image-forming layers are addressed with a thermal printhead or multiple thermal printheads from the opposing surface of the imaging member.
A further object of the invention is to provide a multicolor thermal imaging system wherein at least two different image-forming layers of an imaging member are addressed at least partially independently with a single pass of a thermal printhead.
Another object of the invention is to provide a multicolor thermal imaging system which is capable of providing images which have adequate color separation for a particular application in which the system is used.
Still another object of the invention is to provide novel thermal imaging members.
These and other objects and advantages are accomplished in accordance with the invention by providing a multicolor thermal imaging system wherein at least two, and preferably three, image-forming layers of a thermal imaging member can be addressed at least partially independently, from the same surface of the imaging member, by a single thermal printhead or by multiple thermal printheads. The advantageous thermal imaging system of the invention is based upon at least partially independently addressing a plurality of image-forming layers of a thermal imaging member utilizing two adjustable parameters, namely temperature and time. These parameters are adjusted in accordance with the invention to obtain the desired results in any particular instance by selecting the temperature of the thermal printhead and the period of time for which thermal energy is applied to each of the image-forming layers. According to the invention, each color of the multicolor imaging member can be printed alone or in selectable proportion with the other color(s). Thus, as will be described in detail, according to the invention the temperature-time domain is divided into regions corresponding to the different colors it is desired to combine in a final print.
The image-forming layers of the thermal imaging member undergo a change in color to provide the desired image in the imaging member. The change in color may be from colorless to a color or from colored to colorless or from one color to another color. The term xe2x80x9cimage-forming layerxe2x80x9d as used throughout the application including in the claims, includes all such embodiments. In the case where the change in color is from colorless to a color, an image having different levels of optical density (i.e., different xe2x80x9cgray levelsxe2x80x9d) of that color may be obtained by varying the amount of color in each pixel of the image from a minimum density, Dmin, which is substantially colorless, to a maximum density, Dmax, in which the maximum amount of color is formed. In the case where the change in color is from colored to colorless, different gray levels are obtained by reducing the amount of color in a given pixel from Dmax to Dmin, where ideally Dmin is substantially colorless. In this case, formation of the image involves converting a given pixel from a colored to a less colored, but not necessarily, colorless state.
A number of techniques can be used to achieve the advantageous results provided by exploiting the time and temperature variables in accordance with the invention. These include thermal diffusion with buried layers, chemical diffusion or dissolution in conjunction with timing layers, melting transitions and chemical thresholds. Each of these techniques may be utilized alone, or in combination with others, to adjust the regions of the imaging member in which each desired color will be formed.
In a preferred embodiment, a thermal imaging member includes two, and preferably three, different image-forming layers carried by the same surface of a substrate. In another preferred embodiment, a thermal imaging member includes a layer or layers of image-forming material carried by one surface of a substrate and a layer or layers of image-forming material carried by the opposing surface of the substrate. According to the imaging system of the invention, the image-forming layers of the imaging member can be addressed at least partially independently by a single thermal printhead or multiple printheads in contact with the same surface of the imaging member. In a preferred embodiment, one or two thermal printheads can be utilized to address at least partially independently from one surface of the imaging member two different image-forming layers carried by one surface of the substrate and another thermal printhead utilized to address at least partially independently from the opposing surface of the imaging member one or more image-forming layers carried by the opposing surface of the substrate. The thermal printheads which contact the opposing surfaces of the imaging member can be arranged directly opposite one another or offset from one another such that there is a delay between the times that any discrete area of the imaging member comes into contact with the respective thermal printheads.
In another preferred embodiment one thermal printhead may be used to address at least partially independently two or more different image-forming layers of the imaging member in a single pass and, optionally, a second thermal printhead used to address one or more image-forming layers, either in conjunction with the first thermal printhead, or subsequent thereto.